The present invention relates to a method for correcting the rotational speeds of vehicle wheels determined by wheel sensors, and includes the steps of measuring wheel speeds of the individual wheels of a vehicle, and determining balancing factors of the individual wheels under specific driving conditions, the specific driving conditions being that the vehicle is being neither accelerated nor braked and has no transverse acceleration.
A process of the above type is already known from German Patent Document DE 3,738,914 A1, and according to this process the wheel pair of which the wheel speeds differ the least is determined during slip-free driving. An average value is formed from these wheel speeds, and this average value, when related to the wheel speeds of the other wheels, provides a correction value by means of which the other wheel speeds are corrected. This process is carried out only when no slip occurs on the wheels and when no curve is being negotiated. The occurrence of slip can be detected from the appearance of signals of an anti-lock system (ABS) and/or a drive-slip control (ASR). It is stated in this Patent Document that the steering-angle signal can be evaluated for the purpose of detecting the negotiation of a curve.
An object of the present invention is to provide a process for correcting the rotational speeds of vehicle wheels determined by wheel sensors, in such a way that the correction of the rotational speeds of vehicle wheels determined by wheel sensors is simplified in terms of the signal acquisition.
This and other objects are achieved by the present invention which provides a method for correcting the rotational speeds of vehicle wheels determined by wheel sensors, and includes the steps of measuring wheel speeds of the individual wheels of a vehicle, and determining balancing factors of the individual wheels under specific driving conditions, the specific driving conditions being that the vehicle is being neither accelerated nor braked and has no transverse acceleration. In a first part of the method, the measured wheel speeds are matched under the specific driving conditions and when the vehicle speed is below a first threshold value, the vehicle speed and the specific driving conditions being determined from the measured wheel speeds. The matching of the measured wheel speeds includes the step of determining a matching factor for each of the two vehicle sides so that matched wheel speeds are obtained from the measured wheel speeds that are associated with the matched wheel speeds. One of the matched wheel speeds of the left vehicle side is made equal to its associated measured wheel speed, and the other of the two matched wheel speeds of the left vehicle side is obtained from its associated measured wheel speed by multiplying by the matching factor of the left vehicle side, in such a way that the two matched wheel speeds of the left vehicle side assume the same value. One of the matched wheel speeds of the right vehicle side is made equal to its associated measured wheel speed and the other of the two matched wheel speeds of the right vehicle side is obtained from its associated measured wheel speed by multiplying by the matching factor of the right vehicle side, in such a way that the two matched wheel speeds of the right vehicle side assume the same value.
In a second part of the method, the wheel speeds are measured again to provide newly measured wheel speeds. The newly measured wheel speeds are balanced under the specific driving conditions and when the condition that the vehicle speed is above a second threshold value is satisfied. The newly measured wheel speeds are used to determine, with the matching factors being taken into account, the vehicle speed and the specific driving conditions. The step of balancing includes fixing one of the individual vehicle wheels as a reference wheel and determining a balancing factor for each individual vehicle wheel in relation to this reference wheel, in such a way that the balanced wheel speed coincides with the wheel speed of the reference wheel. Correcting measurements of wheel speeds measured after the balancing step are made by multiplying the wheel speeds measured after the balancing step by the determined balancing factors.
One of the advantages of the present invention in relation to the known state of the art is that there is no need to use a steering-angle sensor. Apart from the wheel speeds, according to the certain embodiments, only signals which can be detected simply and accurately, such as, for example, the appearance of ABS or ASR signals, are used.
In the method according to the present invention, the general driving situation in terms of the factors of vehicle speed v.sub.veh and transverse acceleration a.sub.q is estimated from the order of magnitude and ratio of initially uncorrected wheel speeds .OMEGA..sub.u. Furthermore, an estimation of the drive slip or brake slip .rho. is carried out at the same time. In the range of low transverse accelerations a.sub.q, the vehicle wheels on the individual vehicle sides run approximately on identical path curves, so that the speed of the front wheels .OMEGA..sub.vl, .OMEGA..sub.vr, and rear wheels .OMEGA..sub.hl, .sub.106 .sub.hr on the respective vehicle sides correspond with close approximation to one another when the dynamic wheel radii R.sub.dyn are equal. A further precondition, here, is that a negligible drive and brake slip .rho. should occur.
Since a deviation of the dynamic wheel radii of a plurality of wheels with an increasing vehicle speed v.sub.veh leads to a speed difference .delta..OMEGA. of increasing amount, in the range of low vehicle speeds it is more easily possible to infer transverse acceleration from the difference between the wheel speeds of wheels of one axle, since as a result of different dynamic wheel radii the speed differences assume a smaller amount in this speed range. If a low transverse acceleration is detected in the range of low vehicle speeds, the speeds of the vehicle wheels on the individual vehicle sides are matched respectively at the front and at the rear. This matching can be carried out by providing the wheel speeds of the wheels of the rear axle with a matching factor, in such a way that the speed of the right rear wheel corresponds to the speed of the right front wheel and the speed of the left rear wheel corresponds to the speed of the left front wheel.
Subsequently, in the range of higher vehicle speeds a balancing of the wheel speeds is carried out by fixing the speeds of all the vehicle wheels in relation to a reference wheel during a driving of the vehicle at least virtually free of transverse acceleration. Driving free of transverse acceleration is detected when the difference between the wheel speeds averaged on the two axles becomes equal to 0.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.